Metallic carbide powders are used to make densified or sintered products. For example, it is well-known that monotungsten carbide (WC) is useful in the manufacture of commercially worthwhile items such as cutting tools, tool dies, blast nozzles and drill bits. In producing said WC items, it is common for a tungsten carbide powder to be combined with a metal such as cobalt and, subsequently, densified into a WC/Co cemented carbide by heating when making said tools.
As the particle size of the metallic carbide-metal powder decreases, the densified products generally exhibit improved properties such as increased strength and improved wear resistance. However, due to their high surface energy, if the particles are too small they may cause exaggerated grain growth to occur when forming a cemented carbide part. Exaggerated grain growth adversely affects properties such as strength. Grain growth can be controlled to some extent by addition of grain growth inhibitors such as VC, Cr.sub.3 C.sub.2, or TaC or by starting with a WC having a narrow size distribution.
Densified metallic carbide-metal products having improved properties (e.g., increased strength) are also generally achieved by homogeneously blending the metallic carbide and metal powders. Homogeneously blending the powders generally results in a more uniform microstructure resulting, in less defects such as large grains due to exaggerated grain growth and pores in the densified body.
Monotungsten carbide is typically formed by the carburization of metal tungsten. Metal tungsten carburization processes typically make WC powders having a particle size of about 0.8 micrometer and larger because of the difficulty in producing W metal much smaller than this size. Tungsten metal typically cannot be made much smaller than this size due to synthesis limitations and the tungsten powder being pyrophoric.
Methods which have attempted to make more homogeneously blended and smaller WC-cobalt powder (i.e., WC-cobalt mixed powder) include the following. The article, "Production of WC Powder from WO.sub.3 with Added Co.sub.3 O.sub.4," by Ushijima, et al., published in the Japan Metal Society Journal, 42, No. 9, pages 871-875 (1978), describes a method to produce WC-cobalt powder by carbothermal reduction of WO.sub.3 and Co.sub.3 O.sub.4 in the presence of carbon in the form of carbon black and hydrogen. The WC-cobalt mixed powder formed by this method had a particle size of 0.6 micrometer or greater.
Pollizotti et al., (U.S. Pat. No. 4,851,041) disclose a WC--Co powder produced by reduction decomposition of a suitable mixed metal coordination compound such as tris(ethylenediaminecobalt) tungstate resulting in an atomically mixed high surface area reactive intermediate product, followed by carburization reduction of the intermediate product in flowing CO/CO.sub.2 gas. The WC--Co mixed powder is described as being composed of multiphase composite particles which are larger aggregates containing WC grains (particles) having a size of 10 to 20 nm in a matrix of beta-Co/W/C solid solution.
S. Takatsu in Powder Metallurgy International, Vol. 10, No. 1, pages 13-15, 1978, discloses a method to produce WC powder by reducing a mixed oxide of W and Co by reducing and carburizing with gaseous reagents using a rotary kiln. The mixed oxide is first reduced to metal in a hydrogen atmosphere, then carburized in a methane hydrogen gas mixture, and finally further treated in hydrogen or a methane-hydrogen gas mixture to remove excess carbon and convert W.sub.3 Co.sub.3 C to WC and Co. A homogeneous WC--Co mixed powder is disclosed having a mean particle size of greater than or equal to about 0.4 micrometer.
It is desirable to provide a metallic carbide-metal powder and process to manufacture said powder wherein the powder has a particle size less than about 0.4 micrometer.